Simulcast RF transmission systems involving multiple sites transmitting the same information from each on the same carrier frequency at the same time are generally well known. Such systems are highly desirable since as is well known it is normally not possible for a single RF repeater transmitting site to serve an arbitrarily large geographic coverage area of 800 to 2000 square miles as might be found in a mountainous, rather large county in California, for example. Nevertheless, public service units in such counties require that a mobile unit anywhere in the system be able to call in and receive information from either a control site or another mobile unit in that system. Practical and legal limitations, such as tower height, maximum effective radiated power limitations, natural topographical features which would block the signal transmission to particular areas dictate away from the use of single transmission sites. Accordingly, simulcasting systems are used which involve the technique of transmitting the same information from more than one transmitting site on the same carrier frequency and at the same time so as to extend the coverage area of a base station beyond that which can be obtained by the above noted single site.
The following in an exemplary but not exhaustive listing of prior issued patents describing various aspects of simulcasting in this type of environment:
U.S. Pat. No. 4,696,052 to Breedon
U.S. Pat. No. 4,570,265 to Thro
U.S. Pat. No. 4,516,269 to Krinock
U.S. Pat. No. 4,475,246 to Batlivala et al
U.S. Pat. No. 4,317,220 to Martin
Japanese Patent Disclosure No. 61-107826
FIG. 1 is a schematic diagram of a simplified multiple-site system having three radio repeater (transmitting) sites S1, S2 and S3 providing communications to geographical coverage areas A1, A2 and A3, respectively. A control center or "hub" C (e.g., a dispatch center) provides identical signals to each of sites S1-S3 via links L1-L3, respectively (these links are typically microwave links but can be landline or other type links). Each site S1-S3 transmits the signals it receives from the control center C to its respective coverage area A, so that a mobile or portable transceiver receives the same signal no matter where it happens to be in the communications system overall coverage areas A' (which constitutes the "union", in an analogy to Venn diagrams, of the individual coverage areas A1, A2 and A3).
Mobile or portable transceivers within area A1 can receive the signals transmitted by site S1, transceivers within area S2 can receive the signals transmitted by site S2, and transceivers within area A3 can receive signals transmitted by site S3. Well-known mechanisms are provided in mobile and portable transceivers (and, in some cases, also at the sites) to ensure that transceivers moving out of a first site's coverage area and into a second site's area cease monitoring the signals transmitted by the first site and begin monitoring the signals transmitted by the second site--so that communication is continuously maintained without interruption so long as the transceiver stays within the overall combined system coverage area A'.
In order to prevent "dead zones" from existing at locations between the coverage areas A1-A3, it is desirable to set site transmit effective radiated output power levels (and to geographically locate the sites relative to one another) such that each coverage area slightly overlaps adjacent coverage areas. Overlap regions 012, 013 and 023 shown in the FIG. 1 are examples of such overlap areas. These overlap areas may extend for several miles. Hence, instead of a mobile or portable transceiver receiving no signal at a point effectively "equidistant" (taking effective radiated power into account) between two transmitting sites, the transceiver receives signals from two (or more) sites at the same time. System parameters can be selected so that the transceiver is guaranteed to receive at least one of the signals at a signal strength sufficiently great to overcome noise and Raleigh fading phenomenon and thus provide a useable received signal no matter where in the overlap region the transceiver is located.
While these overlap regions eliminate dead zones, they give rise to another problem: interference between the plural different signals a transceiver may simultaneously receive while it is within an overlap region. Two signals of slightly different RF frequencies produce heterodyning effects (i.e., generation of sum and difference frequencies) in the non-linear detector of a receiver receiving both signals, and may also produce transmit "nulls" (localized dead zones created by interference patterns). The beating and mixing of heterodyning generally must be avoided in a communications system of the type shown in FIG. 1, since it can cause a number of problems (e.g., annoying audible "beat notes" during voice communications), although the complete elimination of heterodyning may be less important in FM (frequency modulation) systems than in AM (amplitude modulation) systems due to the so-called "capture effect" (the FM limiter/detector of an FM receiver "captures" the strongest received signal and is less affected by weaker signals). Prior art solutions to the problems caused by unmatched transmit frequencies include the use of different, spaced-apart transmit frequencies at adjacent sites (undesirable because it requires receivers to alternately lock onto different, separated receive frequencies based on signal strength, a process which takes too much time), randomly varying the transmit frequencies relative to one another to continuously shift the position of interference pattern nulls (see U.S. Pat. No. 4,570,265 to Thro), and synchronizing the transmit frequencies of different sites via a pilot tone originated by a "master" site and transmitted over a voice channel to all of the remote sites (see U.S. Pat. No. 4,317,220 to Martin).
Another serious problem in modern digital FM-based RF communications systems is caused by unequal delay times existing within the system. Referring to FIG. 1, assume a mobile transceiver is located in overlap area 012 and is receiving modulated RF signals transmitted simultaneously by sites S1 and S2. The common signal used to modulate the RF signals transmitted by both site S1 and site S2 originates at control center C and must be transmitted over link L1 to site S1 and over link L2 to site S2. Unfortunately, the delays between the control point C and the inputs to the transmitter modulators of sites S1 and S2 are typically not equal to one another. Moreover, it is not practical to provide links L1-L3 with absolutely identical delay characteristics due to the difference in their physical lengths (the difference may be on the order of miles) and because even identically configured signal processing circuitry at the link ends may exhibit slightly different delay times. In addition, the site transmitter modulation circuits may introduce unequal delays, and further unequal delays exist because of the different RF signal path lengths between the transceiver sites S1 and S2.
Such time delay differences may typically be relatively short (on the order of milliseconds). However, a transceiver located in an overlap region typically alternately receives first one signal and then another signal as the signals fade or the transceiver moves in and out of "shadows" created by obstructions between the transceiver and the transmitting sites (this process of receiving first one signal, then another, and then the one signal again is caused in part by multipath fading effects). Even minor differences in delay times become extremely significant during transmission of digital data or other modulation.
By way of further simplified explanation, nearly everyone while watching television has occasionally come across the same program simulcasted over two different television channels with one version of the program being slightly delayed (e.g, up to several seconds) with respect to the other. It is possible to watch a few seconds of the program on one channel, and quickly change the channel selector to watch the same few seconds again on the other channel. Similarly, a few seconds of the program will actually be "missed" by the viewer if he watches the version of the program which "lags" behind the other version and then quickly switches the program selector to the other channel (which is several seconds "ahead" of the lagging channel).
Now suppose the television receiver regularly, rapidly alternated between the two channels at more or less random times and could not be prevented from doing so (as is the case with a radio transceiver located in an overlap region between two sites of a multisite RF communications system). Needless to say, even voice transmissions would become severely distorted if differential delays of a few milliseconds--let alone seconds--exist in the system. Additionally, high speed digital data becomes severely garbled if it is simulcasted in a system exhibiting more than a few microseconds (millionths of a second) of delay between the time one site transmits a data bit and the time an adjacent site transmits the same data bit.
Fortunately, it is typically possible to minimize time delay differences to on the order of less than a microsecond through various known techniques. For example, it is well known in the art to introduce adjustable delay networks (and phase equalization networks) in line with some or all of links L1-L3 to compensate for inherent different link delay times (see U.S. Pat. No. 4,516,269 to Krinock and U.S. Pat. Nos. 4,696,051 and 4,696,052 to Breeden). Typical conventional digital microwave-multiplex (time domain multiplex) systems exhibit amplitude, phase and delay characteristics that are extremely stable over long periods of time (e.g., many months), so that such additional delays, once adjusted, guarantee that a common signal inputted to all of links L1-L3 at the same time will arrive at the other ends of the links at almost exactly the same time. The same or additional delays can be used to compensate for different, constant delay times introduced by signal processing equipment at the sites S1 and S3 to provide simultaneous coherent transmission of the signals by the different sites.
Analog microwave-multiplex systems (frequency domain multiplex), on the other hand, are effected much more by daily and seasonal propagation variations. Accordingly, if analog microwave systems are employed, frequency simulcast system timing realignment is required, in some cases as often as daily. Attempts to install a simulcast system where analog microwave links exist and replacement costs are prohibitive would for the aforementioned reasons be less than an ideal system. That is to say, after installation of the simulcast system including conventional amplitude and time delay alignment of the system would require frequent realignment. Similar observations may be made with regard to attempts to install a simulcast system where some of the existing sites involve digital microwave links and others involve analog microwave-multiplex systems.
I have discovered that so long as the microwave radio or optical distribution system from the control center to the various sites is phase stable so that the time of arrival of the modulation at one transmit site remains stable relative to the time of arrival of the modulation at any other transmit site, an automatic simulcast system alignment feature may be added whereby automatic periodic alignment is performed. Such alignment will maintain the amplitude and time delay alignment or equalization obtained in the initial manual alignment. This feature is implemented using a single tone on a periodic basis to select and key on each site in a sequential fashion so that the tone is transmitted, received at the control site and processed by computer for comparing it with a reference value and thereafter producing appropriate amplitude and time correction which will compensate for time and amplitude variations in the modulation as distributed to the various simulcast sites. Although auto alignment sequence for each site is implemented on a periodic basis, the alignment mode is activated only where no system activity is sensed. Moreover, during the alignment procedure, if channel activity is sensed, the system switches from alignment mode to normal transmission mode until the system channels are again traffic free. It is, therefore, a primary object of my invention to use a manual alignment procedure to obtain appropriate frequency response and time delay alignment and then implement a periodic auto alignment sequence to maintain the proper alignment.